It wasn't hard to get the pictures. The London area is reported to have 8,000 surveillance cameras watching the streets, although authorities will not confirm the number. People in the United Kingdom are believed to be the most watched in the world. By one estimate, there is one Closed Circuit Television (CCTV) camera for every 14 citizens, so police have countless images from the riots.

"Picture by picture, these criminals are being identified and arrested, and we will not let any phony concerns about human rights get in the way of the publication of these pictures and arrest of these individuals," said Prime Minister David Cameron.

Cameron also said today he would look into whether authorities could limit the use of BlackBerry Messenger, Facebook and Twitter to organize unlawful protests. Police have been forced to concede they had missed messages sent via smartphones after the violence began. He said authorities would investigate "whether it would be right to stop people communicating via these websites and services when we know they are plotting violence, disorder and criminality."

Police invited the public to call if they recognized suspected rioters whose images were posted publicly. Captions on them were cryptic: "A female wearing a vest top who has just carried items from Richer Sounds," or, "A male with grey hooded top and black body warmer about to enter Cash Plus in Thornton Heath."

Facial Recognition

Asking for public help was just one extra step. Face-recognition technology is now inexpensive and widespread, and police could easily use it to match alleged looters' pictures with party photos they might have posted on Facebook or Tumblr.com.

"A lot of these youths are wearing scarves to hide their faces but we're not just reliant on that," Martin Lazell, chairman of the Public CCTV Managers Association in London, said to the Christian Science Monitor. "We can identify people on how they walk, their height, their clothes, shoes, all manner of things."

The possibility of mass unmasking has divided Britons and Americans alike. Many people, angered at the violence, are anxious to see the perpetrators caught. Others say the technology is only as good as the people who use it, and innocent people could be caught in legal nightmares.

On the website Hacker News, people were invited: "Help work out how to do facial recognition on the police photos."

A user replied, "Please do not do this. It compromises people's right to privacy and will artificially incriminate people who may not be participating but are bystanding or trying to get home."

Such arguments will rage as London police pursue one of the largest investigations in recent years.

"If there was ever a need for an evidence-based approach to a social problem, this is it," said Simon Davies, head of a London group called Privacy International. "When Parliament meets to discuss the riots, it should demand evidence to back up any claim of blame, and it should institute a rigorous process to ensure that any response is justified, lawful, viable and fair."